This invention relates to semiconductor packages containing a semiconductor chip to be surface-mounted and method of producing such semiconductor packages.
As an example of semiconductor package to be surface-mounted (hereinafter referred to simply as a semiconductor package or evenly more simply as a package), there have been known structures of the kind having an electrode pattern formed on each of both surfaces of a substrate, a semiconductor chip wire-bonded to one of the surfaces and this surface with the wire-bonded semiconductor chip sealed with a resin material.
In order to mass-produce semiconductor packages of this kind, a method has recently been known to initially prepare an assembly of a plurality of such packages and then separately cut off the individual packages. FIG. 8 shows an outline of such a method, according to which an assembly substrate 100 is initially prepared with a plurality of electrode patterns aligned, each corresponding to a package, and semiconductor chips 11 and 12 set on this assembly substrate 100 by die bonding (FIG. 8 (1)).
Next, each of the semiconductor chips 11 an 12 is wired-bonded (numerals 13 each indicating a wire for the bonding (referred to as the bonding wire) to the electrode pattern on the assembly substrate 1000 ((2) of FIG. 8) and a resin material 104 is applied thereon to seal them in ((3) of FIG. 8).
After an assembly of semiconductor packages is thus prepared, a dicing blade 105 is used to carry out the dicing process for cutting the assembly into separate packages 1 ((4) of FIG. 8).
The next processes for mounting the semiconductor package thus prepared onto a printed circuit board of a target device for the mounting are shown in FIG. 9. Firstly, as shown in (1) of FIG. 9, the process of printing a cream solder 201 on the electrodes (not shown) on the printed circuit board 200 is carried out, followed by the process of mounting the semiconductor packages 1 and other components 106 thereon as shown in (2) of FIG. 9. Thereafter, the circuit board 200 loaded with the components 1 and 106 is placed inside a reflow furnace and undergoes a heating process as shown in (3) of FIG. 9 such that the components inclusive of the semiconductor packages 1 come to be soldered onto the printed substrate 200.
Thus, semiconductor packages can be mounted to a printed circuit board by the same method as ordinary components, but since the substrate, the semiconductor chips and the resin have different thermal expansion coefficients, faulty conditions such as cracks in the resin may occur by the reflow, and the packages may end up by exploding in a worst situation, as reported in Japanese Patent Publication Tokkai 2000-124363.
Since a gold wire is usually used for the wire-bonding of semiconductor packages, furthermore, a gold plating process is carried out also on the electrode patterns on the circuit board in order to increase the strength of the wire bonding. It is also known that a film with a thickness appropriate for wire bonding can be obtained by an electrolytic gold plating process. Japanese Patent Publication Tokkai 2001-110940, for example, discloses that an improved contacting characteristic in wire bonding was observed by forming a nickel film on the electrode pattern on the substrate by electrolytic plating and then forming a gold film thereover.
For producing a semiconductor package assembly as described above, it is preferable to carry out an electrolytic gold plating process prior to the die bonding process as shown in (1) of FIG. 8. For this reason, it has been known to connect the electrode patterns in each area by means of a linear electrode (referred to as a lead-in wire) such that an electrolytic gold plating process can be carried out together at once.
FIG. 10 shows an example of structure of a single semiconductor package 1 produced by the production method illustrated by FIG. 8. This package 1 corresponds to just one of the pieces that have been separated by the dicing process, numeral 10 therein indicating an individually separated substrate portion (also referred to as an interposer), numerals 11, 12 and 13 again indicating semiconductor chips and bonding wires and numeral 14 indicating the sealing resin material, as in FIG. 8.
Numerals 15 each indicate an electrode pattern on the mounting surface of the semiconductor chip to which is continuously formed a lead-in wire 16 for electrolytic gold plating. Neighboring ones of these lead-in wires 16 were originally connected to each other on the assembly substrate 100 but are now as shown surrounded by circles of broken lines after the original assembly substrate 100 was sealed with resin and was subjected to the dicing process, being exposed at edge surfaces of the package 1 where they are sandwiched between the resin material 14 and the substrate 10.
Since these lead-in wires 16 for electrolytic gold plating are exposed on side surfaces between the resin 14 and the substrate 10, as described above, they are in a condition where water components from outside can easily taken in by the resin because the contacting condition between the gold plate layer and the resin is not favorable. This is why the probability of occurrence of faults such as cracks and explosion of packages as explained above tends to increase.
Examples of prior art method for preventing the hygroscopic property of a package from increasing include including an additive to the sealing resin to thereby adjust its thermal expansion rate and hydroscopic property. For applications where the transparency characteristic of the package needs to be maintained such as semiconductor packages for light emitter and receiver of an optical sensor (such as those incorporating a photo IC or a light-emitting diode), however, it is not an easy solution to use an additive because the additive may adversely affect the transparency characteristic of the resin.
On the other hand, electroless plating process may be considered instead of electrolytic plating such that lead-in wires for plating may be dispensed with. In order to obtain by electroless plating a metal film thick enough for providing sufficiently strong wire bonding, however, the plating process will have to be repeated many times and hence the process becomes troublesome.